Conventionally, in order to prevent burning of a piston in an engine during operation, there are attempts to measure a thickness of a film of lubricating oil. One of the methods of measuring a thickness of an oil film is a method of measuring a capacitance of a gap between a sliding surface of a piston ring in a cylinder liner and the piston ring by use of a detection electrode, which is provided on the sliding surface of the piston ring, and then calculating an oil film thickness of a lubricating oil.
For example, as a prior art document that discloses a capacitance-detection-type film thickness measuring technique as described above, Patent Document 1 as shown below is set forth.    Patent Document 1: Japanese Unexamined Patent Application, First Publication No. 2007-107947
However, the aforementioned conventional technique requires an electronic circuit that, in order to measure the capacitance, is used for the purposes such as supplying charging electricity to the detection electrode, detecting the period of time for charging, and discharging the electricity charged in the detection electrode. This increases the cost of the apparatus.
Furthermore, the aforementioned technique requires that a through-hole be bored in the cylinder and cylinder liner by machining, and that a detection electrode be installed at a forward end portion (a sliding surface of the piston ring) of the through-hole. Therefore, the detection electrode is installed at much expense in time and effort. In addition, there is a possibility of an occurrence of combustion gas leakage through the through-hole. Consequently, there is a demand for development of a technique that detects a sign of burning of a piston by a less expensive and simpler method.
The present invention has been achieved in view of the aforementioned problems, and has objects as follows:
(1) To detect a sign of burning of a piston by use of an apparatus less expensive than conventional apparatuses.
(2) To detect a sign of burning of a piston by a method of installing a sensor simpler than conventional methods.